Deformation behaviors during hot compression of GH4700 alloy for 700℃ ultra-supercritical boilers

The hot deformation behavior of nickel-based superalloy GH4700 was investigated by isothermal compression tests at temperatures of 1120 to 1210℃ and strain rates of 0.1 to 20 s^-1 with deformations of 15% to 60%. The flow curves at high strain rate and low temperature were corrected in consideration of the deformation-heating effect. Accurate constitutive equations were established between peak stress and deformation parameters, i.e., temperature and strain rate. The activation energy of the alloy was determined to be 322 kJ. Microstructure analysis results show that dynamic recrystallization (DRX) is the principal softening mechanism during hot working. Strain-induced-grain-boundary-migration is the nucleation mechanism, which is promoted by the increase of both temperature and strain rate. The ratio of DRX grains is increased by temperature and strain, while the iso-ratio contour of DRX exhibits an "anti-C" type in the strain rate-temperature coordinate system. The relationship between DRX grain size and deformation parameters was calculated to be a piecewise function which depends on strain rate.